The present invention relates to the production of hydrocarbons from earth formations, and more specifically to a new method for determining the amount of oil in such earth formations.
The petroleum industry commonly relies on obtaining core from an earth formation to measure the amount of hydrocarbons contained in the formation. The three principal types of coring presently practiced are conventional core, pressure core, and sponge core. Conventional core is the least expensive means of coring, but fluids and gases present in the formation are displaced from the core as it is brought to the surface. These "blowdown" losses can be a significant fraction of the fluid contained in the core, and thus the oil saturation determined on conventional core will be too low.
Pressure core eliminates "blowdown losses" by maintaining the core at reservoir pressures until analyzed in the laboratory. However, pressure coring, which is no longer widely available as a commercial service, is quite expensive, typically ten times more expensive than conventional core. Moreover, core recovery from pressure coring operations tends to be lower than with conventional coring.
Recently, a third method of coring has been developed, called sponge coring. In this method, the "blowdown losses" from conventional core are trapped in an oil-wet polyurethane sponge lining on the inner barrel of a conventional core barrel. Sponge coring is substantially less expensive than pressure coring and often results in superior core recovery as well. Sponge coring has become the fastest growing section of the coring industry.
Once sponge core arrives in the laboratory, it is necessary to measure accurately the amount of hydrocarbons trapped in the polyurethane sponge. This has proven surprisingly difficult, as explained in greater detail in the above-noted '622 application. Various core laboratories have tried mechanical and pressurized solvent methods for extracting the sponge, but none has yielded an accurate or precise method for determining the oil volumes.
The above-mentioned patent '622 application describes a method for extracting oil from a polyurethane sponge without substantially affecting the sponge by using a low boiling point solvent selected from among the class of cycloalkanes, ethers, and Freons. With these low boiling point solvents, the solvent can simply be evaporated away after extraction. Alternatively, the patent application discloses that a test solution of oil removed from the sponge can then be compared with a standard solution using near infrared (IR) spectroscopy or supercritical fluid chromatography. Near IR spectroscopy measures the number of C--H bonds in the sample, while supercritical fluid chromatography measures the aromatic concentration in the solution. Among the solvents specifically disclosed in that patent application are the cycloalkanes: cyclobutane, cyclopentane, cyclohexane; the ether; diethyl ether; and the Freons: Freon-11, and Freon-114.
The new methods thus described in the above-noted patent applications have significant advantages over past technologies. Nevertheless, it has been found that, although the above-mentioned solvents do not substantially dissolve the sponge, there are nonetheless some unpolymerized polyurethane precursors that are extracted by these solvents. Polyurethane is made by polymerization of a diisocyanate and a diol. These chemical structures have been identified in significant quantity (compared to the dissolved oil) by .sup.1 H and .sup.13 C nuclear magnetic resonance studies of extracts using the Freon-11 solvent of the above-mentioned method. This necessitates, in the presence of the dissolved polyurethane components, several additional stages of distillation, centrifugation, and emulsion breaking to practice the step of evaporating the solvent from the oil. Moreover, the separation between the extracted sponge layer and the oil/solvent is, even then, not always clear, particularly at low oil concentration. This can lead to errors in volumetric determination of the oil content.
A need therefore remains for a new and improved method and apparatus for use in determining the oil saturation of an earth formation by means of sponge coring, which method and apparatus are not subject to the several limitations discussed above. More particularly, a need remains for a method and apparatus for performing such analyses which can readily accommodate and account for these small amounts of dissolved unpolymerized polyurethane precursors in the oil/solvent mixture, essentially regardless of concentration. A need also remains for such a method and apparatus which can preferably perform the required analysis quickly and accurately, ideally without the time-consuming need to evaporate part of the solvent and/or to separate out the dissolved unpolymerized polyurethane precursors. The method and apparatus should also be uncomplicated, versatile, and reliable, inexpensive to implement and perform, and readily suited to the widest possible utilization in determining oil saturations of earth formations by means of sponge coring.